Exhaust emission purification apparatus of compression ignition type internal combustion engine

ABSTRACT

A fuel adding valve ( 14 ), an HC adsorbing and oxidation catalyst ( 11 ), and a NO x  storing catalyst ( 12 ) are successively arranged in an exhaust passage of an internal combustion engine toward the downstream side. When the NO x  storing catalyst ( 12 ) should release NO x , particulate fuel is added from the fuel adding valve ( 14 ). This fuel is adsorbed once at the HC adsorbing and oxidation catalyst ( 11 ), then gradually evaporates to make the air-fuel ratio of the exhaust gas flowing into the NO x  storing catalyst ( 12 ) rich. Due to this, NO x  is released from the NO x  storing catalyst ( 12 ).

TECHNICAL FIELD

The present invention relates to an exhaust purification device of acompression ignition type internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine having arranged in anengine exhaust passage an NO_(x) storing catalyst which stores NO_(x)contained in exhaust gas when the air-fuel ratio of the inflowingexhaust gas is lean and releases the stored NO_(x) when the oxygenconcentration in the inflowing exhaust gas falls. In this internalcombustion engine, the NO_(x) produced when burning fuel under a leanair-fuel ratio is stored in the NO_(x) storing catalyst.

However, when using such an NO_(x) storing catalyst, it is necessary tomake the NO_(x) storing catalyst release the NO_(x) before the NO_(x)storing capability of the NO_(x) storing catalyst becomes saturated. Inthis case, if making the air-fuel ratio of the exhaust gas flowing intothe NO_(x) storing catalyst rich, it is possible to make the NO_(x)storing catalyst release the NO_(x) and to reduce the released NO_(x).Therefore, in conventional internal combustion engines, the NO_(x)storing catalyst is made to release NO_(x) by making the air-fuel ratioin the combustion chamber rich or by feeding fuel into the engineexhaust passage upstream of the NO_(x) storing catalyst to make theair-fuel ratio of the exhaust gas flowing into the NO_(x) storingcatalyst rich.

However, to make an NO_(x) storing catalyst release NO_(x) well,sufficiently gasified rich air-fuel ratio exhaust gas has to be made toflow into the NO_(x) storing catalyst. In this case, if making theair-fuel ratio in the combustion chamber rich, the sufficiently gasifiedrich air-fuel ratio exhaust gas flows into the NO_(x) storing catalyst,so it is possible to make the NO_(x) storing catalyst release the NO_(x)well. However, if making the air-fuel mixture in the combustion chamberrich, there is the problem that a large amount of soot is produced.Further, if injecting additional fuel into the expansion stroke orexhaust stroke so as to make the air-fuel ratio of the exhaust gasexhausted from the combustion chamber rich, the injected fuel sticks tothe inside walls of the cylinder bore, i.e., bore flushing occurs.

As opposed to this, when injecting fuel into the engine exhaust passageupstream of an NO_(x) storing catalyst, the problems of soot beingproduced or bore flushing occurring as explained above no longer arise.However, when injecting fuel into the engine exhaust passage upstream ofthe NO_(x) storing catalyst, there is the problem that the injected fuelis not sufficiently gasified and therefore the NO_(x) storing catalystcannot be made to release NO_(x) well.

On the other hand, known in the art is an internal combustion enginearranging a hydrocarbon, that is, HC adsorbing catalyst for adsorbing HCcontained in exhaust gas in the engine exhaust passage upstream of theNO_(x) storing catalyst (see Japanese Unexamined Patent Publication(Kokai) No. 2003-97255). In this internal combustion engine, the HCproduced when burning fuel under a lean air-fuel ratio is adsorbed bythe HC adsorbing catalyst and the NO_(x) produced at that time is storedin the NO_(x) storing catalyst.

However, in this internal combustion engine, when the temperature of theHC adsorbing catalyst becomes near the activation temperature, that is,near 200° C., the oxidation reaction of the adsorbed HC becomes activeand as a result the oxygen in the exhaust gas is rapidly consumed, sothe oxygen concentration in the exhaust gas rapidly falls. Therefore, atthis time, if additionally supplying a small amount of fuel, it ispossible to make the air-fuel ratio of the exhaust gas rich. Therefore,in this internal combustion engine, it is detected whether a sufficientamount of oxygen has been consumed at the HC adsorbing catalyst, and theair-fuel ratio of the exhaust gas is made rich when a sufficient amountof oxygen is being consumed in the HC adsorbing catalyst so as to makethe NO_(x) storing catalyst release NO_(x).

However, in this internal combustion engine, the air-fuel ratio in thecombustion chamber is made rich. Fuel is not injected into the engineexhaust passage. Therefore, the above problem arises. Further, in thisinternal combustion engine, the period when the temperature of the HCadsorbing catalyst becomes near the activation temperature, that is, theperiod when a sufficient amount of oxygen is consumed in the HCadsorbing catalyst, is limited, so the temperature of the HC adsorbingcatalyst will not become the activation temperature in the periodrequired as seen from the action of the NO_(x) storing catalystreleasing the NO_(x) and consequently there is the problem that theNO_(x) storing catalyst cannot release NO_(x) when the NO_(x) storingcatalyst has to release the NO_(x).

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purificationdevice of a compression ignition type internal combustion enginedesigned to enable an NO_(x) storing catalyst to release NO_(x) welleven when feeding fuel into the engine exhaust passage upstream of theNO_(x) storing catalyst so as to make the NO_(x) storing catalystrelease NO_(x).

To achieve the above object, according to the present invention,provision is made of fuel adding means for adding particulate fuel intoexhaust gas, an HC adsorbing and oxidation catalyst arranged in anengine exhaust passage downstream of the fuel adding means for adsorbingand oxidizing hydrocarbons contained in the exhaust gas, and an NO_(x)storing catalyst arranged in the engine exhaust passage downstream ofthe HC adsorbing and oxidation catalyst for storing NO_(x) contained inthe exhaust gas when the air-fuel ratio of the inflowing exhaust gas islean and releasing the stored NO_(x) when the air-fuel ratio of theinflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich,particulate fuel is added from the fuel adding means when making theair-fuel ratio of the exhaust gas flowing into the NO_(x) storingcatalyst rich to make the NO_(x) storing catalyst release NO_(x), theamount of addition of particulate fuel at this time is set to an amountwhereby the air-fuel ratio of the exhaust gas flowing into the HCabsorbing and oxidation catalyst becomes a rich air-fuel ratio smallerthan the rich air-fuel ratio when flowing into the NO_(x) storingcatalyst, and after the added particulate fuel is adsorbed at the HCadsorbing and oxidation catalyst, the majority of the adsorbed fuel isoxidized in the HC adsorbing and oxidation catalyst and the air-fuelratio of the exhaust gas flowing into the NO_(x) storing catalyst ismade rich over a longer period than when the air-fuel ratio of theexhaust gas flowing into the HC adsorbing and oxidation catalyst is maderich.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of a compression ignition type internal combustionengine.

FIG. 2 is an overview of another embodiment of a compression ignitiontype internal combustion engine.

FIG. 3 gives views of the structure of a particulate filter.

FIG. 4 is a sectional view of a surface part of a catalyst carrier of anNO_(x) storing catalyst.

FIG. 5 is a side sectional view of an HC adsorbing and oxidationcatalyst.

FIG. 6 is a sectional view of a surface part of a catalyst carrier of anHC adsorbing and oxidation catalyst.

FIG. 7 is a view of an amount of fuel adsorption.

FIG. 8 is a view of the change in the air-fuel ratio of exhaust gas.

FIG. 9 is a view of the relationship between a fuel addition time and anair-fuel ratio A/F of exhaust gas, a temperature rise ΔT, exhausted HCamount G, and a rich time.

FIG. 10 is a view of the change in the air-fuel ratio of exhaust gas.

FIG. 11 is a view of an amount of fuel addition.

FIG. 12 is a view of NO_(x) release control.

FIG. 13 is a view of a map etc. of a stored NO_(x) amount NOXA.

FIG. 14 is a flow chart of exhaust purification processing.

FIG. 15 is a flow chart of fuel addition processing.

FIG. 16 is a flow chart of fuel addition processing.

FIG. 17 is a flow chart of fuel addition processing.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an overview of a compression ignition type internalcombustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamberof each cylinder, 3 an electronically controlled fuel injector forinjecting fuel into each combustion chamber 2, 4 an intake manifold, and5 an exhaust manifold. The intake manifold 4 is connected through anintake duct 6 to an outlet of a compressor 7 a of an exhaustturbocharger 7. The inlet of the compressor 7 a is connected to an aircleaner 8. Inside the intake duct 6 is arranged a throttle valve 9driven by a step motor. Further, around the intake duct 6 is arranged acooling device 10 for cooling the intake air flowing through the insideof the intake duct 6. In the embodiment shown in FIG. 1, the enginecooling water is guided into the cooling device 10. The engine coolingwater cools the intake air. On the other hand, the exhaust manifold 5 isconnected to an inlet of an exhaust turbine 7 b of the exhaustturbocharger 7, while the outlet of the exhaust turbine 7 b is connectedto an inlet of an HC adsorbing and oxidation catalyst 11. Further, theoutlet of the HC adsorbing and oxidation catalyst 11 is connectedthrough an exhaust pipe 13 to an NO_(x) storing catalyst 12. The exhaustmanifold 5 is provided with a fuel adding valve 14 for adding miststate, that is, particulate state fuel into the exhaust gas. In thisembodiment of the present invention, this fuel is diesel oil.

The exhaust manifold 5 and the intake manifold 4 are interconnectedthrough an exhaust gas recirculation (hereinafter referred to as an“EGR”) passage 15. The EGR passage 15 is provided with an electronicallycontrolled EGR control valve 16. Further, around the EGR passage 15 isarranged a cooling device 17 for cooling the EGR gas flowing through theinside of the EGR passage 15. In the embodiment shown in FIG. 1, theengine cooling water is guided into the cooling device 17. The enginecooling water cools the EGR gas. On the other hand, each fuel injector 3is connected through a fuel feed tube 18 to a common rail 19. Thiscommon rail 19 is supplied with fuel from an electronically controlledvariable discharge fuel pump 20. The fuel supplied into the common rail19 is supplied through each fuel feed tube 18 to the fuel injector 3.

An electronic control unit 30 is comprised of a digital computerprovided with a ROM (read only memory) 32, a RAM (random access memory)33, a CPU (microprocessor) 34, an input port 35, and an output port 36all connected to each other by a bidirectional bus 31. The inlet of theHC adsorbing and oxidation catalyst 11 is provided with a temperaturesensor 21 for detecting the temperature of the exhaust gas flowing intothe HC adsorbing and oxidation catalyst 11, while the exhaust passage 13is provided with a temperature sensor 22 for detecting the temperatureof the exhaust gas flowing out from the HC adsorbing and oxidationcatalyst 11. The output signals of the temperature sensors 21 and 22 areinput through corresponding AD converters 37 to the input port 35.Further, the NO_(x) storing catalyst 12 is provided with a differentialpressure sensor 23 for detecting the differential pressure before andafter the NO_(x) storing catalyst 12. The output signal of thedifferential pressure sensor 23 is input through the corresponding ADconverter 37 to the input port 35.

An accelerator pedal 40 has a load sensor 41 generating an outputvoltage proportional to the amount of depression L of the acceleratorpedal 40 connected to it. The output voltage of the load sensor 41 isinput through a corresponding AD converter 37 to the input port 35.Further, the input port 35 has a crank angle sensor 42 generating anoutput pulse each time the crankshaft turns for example by 15 degreesconnected to it. On the other hand, the output port 36 is connectedthrough corresponding drive circuits 38 to the fuel injectors 3,throttle valve 9 step motor, fuel adding valve 14, EGR control valve 16,and fuel pump 20.

FIG. 2 shows another embodiment of a compression ignition type internalcombustion engine. In this embodiment, the HC adsorbing and oxidationcatalyst 11 is provided with a temperature sensor 25 for detecting thetemperature of the HC adsorbing and oxidation catalyst 11, while theexhaust passage 24 connected to the outlet of the NO_(x) storingcatalyst 12 is provided inside it with an air-fuel ratio sensor 26 fordetecting the air-fuel ratio of the exhaust gas.

First, explaining the NO_(x) storing catalyst 12 shown in FIG. 1 andFIG. 2, the NO_(x) storing catalyst 12 is carried on a three-dimensionalmesh structure monolith carrier or pellet carriers or is carried on ahoneycomb structure particulate filter. In this way, the NO_(x) storingcatalyst 12 can be carried on various types of carriers, but below, theexplanation will be made of the case of carrying the NO_(x) storingcatalyst 12 on a particulate filter.

FIGS. 3(A) and (B) show the structure of the particulate filter 12 acarrying the NO_(x) storing catalyst 12. Note that FIG. 3(A) is a frontview of the particulate filter 12 a, while FIG. 3(B) is a side sectionalview of the particulate filter 12 a. As shown in FIGS. 3(A) and (B), theparticulate filter 12 a forms a honeycomb structure and is provided witha plurality of exhaust flow passages 60 and 61 extending in parallelwith each other. These exhaust flow passages are comprised by exhaustgas inflow passages 60 with downstream ends sealed by plugs 62 andexhaust gas outflow passages 61 with upstream ends sealed by plugs 63.Note that the hatched portions in FIG. 3(A) show plugs 63. Therefore,the exhaust gas inflow passages 60 and the exhaust gas outflow passages61 are arranged alternately through thin wall partitions 64. In otherwords, the exhaust gas inflow passages 60 and the exhaust gas outflowpassages 61 are arranged so that each exhaust gas inflow passage 60 issurrounded by four exhaust gas outflow passages 61, and each exhaust gasoutflow passage 61 is surrounded by four exhaust gas inflow passages 60.

The particulate filter 12 a is formed from a porous material such as forexample cordierite. Therefore, the exhaust gas flowing into the exhaustgas inflow passages 60 flows out into the adjoining exhaust gas outflowpassages 61 through the surrounding partitions 64 as shown by the arrowsin FIG. 3(B).

When the NO_(x) storing catalyst 12 is carried on the particulate filter12 a in this way, the peripheral walls of the exhaust gas inflowpassages 60 and exhaust gas outflow passages 61, that is, the surfacesof the two sides of the partitions 64 and inside walls of the fine holesof the partitions 64, carry a catalyst carrier comprised of alumina.FIGS. 4(A) and (B) schematically show the cross-section of the surfacepart of this catalyst carrier 45. As shown in FIGS. 4(A) and (B), thecatalyst carrier 45 carries a precious metal catalyst 46 diffused on itssurface. Further, the catalyst carrier 45 is formed with a layer of anNO_(x) absorbent 47 on its surface.

In this embodiment of the present invention, platinum Pt is used as theprecious metal catalyst 46. As the ingredient forming the NO_(x)absorbent 47, for example, at least one element selected from potassiumK, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Ca,or another alkali earth, lanthanum La, yttrium Y, or another rare earthmay be used.

If the ratio of the air and fuel (hydrocarbons) supplied to the engineintake passage, combustion chambers 2, and exhaust passage upstream ofthe NO_(x) storing catalyst 12 is referred to as the “air-fuel ratio ofthe exhaust gas”, the NO_(x) absorbent 47 performs an NO_(x) absorptionand release action of storing the NO_(x) when the air-fuel ratio of theexhaust gas is lean and releasing the stored NO_(x) when the oxygenconcentration in the exhaust gas falls.

That is, if explaining this taking as an example the case of usingbarium Ba as the ingredient forming the NO_(x) absorbent 47, when theair-fuel ratio of the exhaust gas is lean, that is, when the oxygenconcentration in the exhaust gas is high, the NO contained in theexhaust gas is oxidized on the platinum Pt 46 such as shown in FIG. 4(A)to become NO₂, then is absorbed in the NO_(x) absorbent 47 and diffusesin the NO_(x) absorbent 47 in the form of nitric acid ions NO₃ ⁻ whilebonding with the barium oxide BaO. In this way, the NO_(x) is absorbedin the NO_(x) absorbent 47. So long as the oxygen concentration in theexhaust gas is high, NO₂ is produced on the surface of the platinum Pt46. So long as the NO_(x) absorbing capability of the NO_(x) absorbent47 is not saturated, the NO₂ is absorbed in the NO_(x) absorbent 47 andnitric acid ions NO₃ ⁻ are produced.

As opposed to this, by making the air-fuel ratio of the exhaust gas richor the stoichiometric air-fuel ratio, since the oxide concentration inthe exhaust gas falls, the reaction proceeds in the reverse direction(NO₃ ⁻→NO₂) and therefore, as shown in FIG. 4(B), the nitric acid ionsNO₃ ⁻ in the NO_(x) absorbent 47 are released from the NO_(x) absorbent47 in the form of NO₂. Next, the released NO_(x) is reduced by theunburned hydrocarbons or CO included in the exhaust gas.

In this way, when the air-fuel ratio of the exhaust gas is lean, thatis, when burning fuel under a lean air-fuel ratio, the NO_(x) in theexhaust gas is absorbed in the NO_(x) absorbent 47. However, ifcontinuing to burn fuel under a lean air-fuel ratio, during that timethe NO_(x) absorbing capability of the NO_(x) absorbent 47 will end upbecoming saturated and therefore NO_(x) will end up no longer being ableto be absorbed by the NO_(x) absorbent 47. Therefore, in this embodimentaccording to the present invention, before the absorbing capability ofthe NO_(x) absorbent 47 becomes saturated, a reducing agent is suppliedfrom the reducing agent supply valve 14 so as to temporarily make theair-fuel ratio of the exhaust gas rich and thereby release the NO_(x)from the NO_(x) absorbent 47.

Now, as explained above, if adding fuel from the fuel adding valve 14 tomake the air-fuel ratio of the exhaust gas rich, the NO_(x) absorbent 47releases NO_(x) and the released NO_(x) is reduced by the unburned HCand CO contained in the exhaust gas. In this case, if the added fuel isin the liquid state, theoretically even if the air-fuel ratio of theexhaust gas becomes rich, the NO_(x) absorbent 47 will not releaseNO_(x). Further, when the fuel is in the liquid state, the NO_(x) willnot be reduced. That is, to make the NO_(x) absorbent 47 release NO_(x)and to reduce the released NO_(x), it is necessary to make the air-fuelratio of the gaseous ingredients in the exhaust gas flowing into theNO_(x) storing catalyst 12 rich.

In the present invention, the fuel added from the fuel adding valve 14is in the particulate state. Part of the fuel becomes a gaseous, but themajority is in the liquid state. In the present invention, even if themajority of the fuel added is in the liquid state, the HC adsorbing andoxidation catalyst 11 is arranged upstream of the NO_(x) storingcatalyst 12 so that the fuel flowing into the NO_(x) storing catalyst 12becomes gaseous. Next, the HC adsorbing and oxidation catalyst 11 willbe explained.

FIG. 5 is a side sectional view of the HC adsorbing and oxidationcatalyst 11. As shown in FIG. 5, the HC adsorbing and oxidation catalyst11 forms a honeycomb structure and provides a plurality of exhaust gaspassages 65 extending straight. The HC adsorbing and oxidation catalyst11 is formed from a material with a large relative surface area having aporous structure such as zeolite. The base of the HC adsorbing andoxidation catalyst 11 shown in FIG. 5 is made of a type of zeolite, thatis, mordenite. FIGS. 6(A) to (D) schematically show cross-sections ofthe surface part of the HC adsorbing and oxidation catalyst 11. Notethat FIG. 6(B) shows an enlarged view of the part B in FIG. 6(A), FIG.6(C) shows the same cross-section as FIG. 6(B), and FIG. 6(D) shows anenlarged view of the part D in FIG. 6(C). As will be understood fromFIGS. 6(B) and (C), the surface of the HC adsorbing and oxidationcatalyst 11 forms a relief, rough surface shape. On the surface havingthis rough surface shape, as shown in FIG. 6(D), a large number of finepores 51 are formed and a precious metal catalyst 52 made of platinum Ptis carried dispersed.

When particulate fuel is added from the fuel adding valve 14, part ofthe fuel evaporates and becomes gaseous, but the majority is adsorbed onthe surface of a base 50 in the form of particles. FIGS. 6(A) and (B)show the state of adsorption of the fuel particles 53. The ratio ofadsorption of fuel when fuel is adsorbed in the liquid state becomesconsiderably high compared with the ratio of adsorption of gaseous fuel.Note that the amount of adsorption of the particulate fuel which the HCadsorbing and oxidation catalyst 11 is able to adsorb, as shown in FIG.7(A), becomes greater the lower the temperature of the HC adsorbing andoxidation catalyst 11. Further, if the spatial velocity of the flow ofexhaust gas in the HC adsorbing and oxidation catalyst 11 becomesfaster, that is, if the flow rate of the exhaust gas becomes faster, theamount of the fuel added from the fuel adding valve 14 which is gasifiedand the amount of the particulate fuel passing straight through theexhaust passages 65 in the HC adsorbing and oxidation catalyst 11 willincrease. Therefore, the amount of adsorption of the particulate fuelwhich the HC adsorbing and oxidation catalyst 11 can adsorb, as shown inFIG. 7(B), decreases the faster the spatial velocity.

Next, as shown in FIGS. 6(C) and (D), the fuel particles 53 adsorbed onthe surface of the base 50 gradually evaporate to form gaseous fuel.This gaseous fuel is mainly comprised of HC with a large number ofcarbon atoms. The HC with the large number of carbon atoms is cracked atthe acid points on the surface of the zeolite or on the precious metalcatalyst 52 and converted to HC with a small number of carbon atoms. Theconverted gaseous HC immediately reacts with the oxygen in the exhaustgas to be oxidized. The majority of the fuel particles 53 adsorbed onthe surface of the base 50 reacts with the oxygen in the exhaust gas, soalmost all of the oxygen contained in the exhaust gas is consumed. As aresult, the oxygen concentration in the exhaust gas falls and the NO_(x)storing catalyst 12 releases the NO_(x).

On the other hand, at this time, the exhaust gas contains residualgaseous HC, so the air-fuel ratio of the exhaust gas becomes rich. Thisgaseous HC flows into the NO_(x) storing catalyst 12, where the gaseousHC reduces the NO_(x) released from the NO_(x) storing catalyst 12.

FIG. 8 shows the amount of addition of fuel from the fuel adding valve14 and the air-fuel ratio A/F of the exhaust gas at the time of enginelow speed, low load operation. Note that in FIG. 8, (A) shows theair-fuel ratio A/F of the exhaust gas flowing into the HC adsorbing andoxidation catalyst 11, (B) shows the air-fuel ratio A/F of the exhaustgas flowing out from the HC adsorbing and oxidation catalyst 11 andflowing into the NO_(x) storing catalyst 12, and (C) shows the air-fuelratio A/F of the exhaust gas flowing out from the NO_(x) storingcatalyst 12.

In this embodiment of the present invention, when the NO_(x) storingcatalyst 12 should release NO_(x), as shown in FIG. 8, a drive signalcomprised of a plurality of continuous pulses is supplied to the fueladding valve 14. At this time, in actuality, the fuel continues to becontinuously added while these continuous pulses are supplied. Whilefuel is being supplied from the fuel adding valve 14, the air-fuel ratioof the exhaust gas flowing into the HC adsorbing and oxidation catalyst11, as shown in FIG. 8(A), becomes a considerably rich air-fuel ratio ofup to 5.

On the other hand, when fuel is added from the fuel adding valve 14, thefuel particles are adsorbed on the HC adsorbing and oxidation catalyst11, then the fuel gradually evaporates from the fuel particles and, asexplained above, is cracked and reformed. Part of the fuel evaporatedfrom the fuel particles or the reformed fuel reacts with the oxygencontained in the exhaust gas to be oxidized, whereby the oxygenconcentration in the exhaust gas falls. On the other hand, the excessfuel, that is, the excess HC is exhausted from the HC adsorbing andoxidation catalyst 11. As a result, the air-fuel ratio A/F of theexhaust gas flowing out from the HC adsorbing and oxidation catalyst 11becomes just slightly rich. That is, the fuel gradually evaporates fromthe fuel particles adsorbed on the HC adsorbing and oxidation catalyst11 and the air-fuel ratio A/F of the exhaust gas flowing out from the HCadsorbing and oxidation catalyst 11 continues to be just slightly richuntil the amount of the adsorbed fuel particles becomes small.Therefore, as shown in FIG. 8(B), the air-fuel ratio A/F of the exhaustgas flowing out from the HC adsorbing and oxidation catalyst 11continues to be just slightly rich over a considerable time after theaction of addition of fuel from the fuel adding valve 14 ends.

If the air-fuel ratio A/F of the exhaust gas flowing out from the HCadsorbing and oxidation catalyst 11 and flowing into the NO_(x) storingcatalyst 12 becomes rich, NO_(x) is released from the NO_(x) storingcatalyst 12 and the released NO_(x) is reduced by the unburned HC andCO. In this case, as explained above, the unburned HC flowing into theNO_(x) storing catalyst 12 is reformed at the HC adsorbing and oxidationcatalyst 11. Therefore, the released NO_(x) is reduced well by theunburned HC. As will be understood from FIG. 8(C), while the action ofrelease of NO_(x) from the NO_(x) storing catalyst 12 and the action ofreduction are performed, the air-fuel ratio A/F of the exhaust gasflowing out from the NO_(x) storing catalyst 12 is maintained atsubstantially the stoichiometric air-fuel ratio.

In this way, in the present invention, when making the air-fuel ratio ofthe exhaust gas flowing into the NO_(x) storing catalyst 12 rich so asto make the NO_(x) storing catalyst 12 release NO_(x), particulate fuelis added from the fuel adding valve 14. The amount of addition of theparticulate fuel at this time is set to an amount so that the air-fuelratio of the exhaust gas flowing into the HC adsorbing and oxidationcatalyst 11 becomes a rich air-fuel ratio smaller than the rich air-fuelratio when flowing into the NO_(x) storing catalyst 12, in the exampleshown in FIG. 8, less than half of that rich air-fuel ratio.

On the other hand, the particulate fuel added at this time is adsorbedon the HC adsorbing and oxidation catalyst 11, then the majority of theadsorbed fuel is oxidized in the HC adsorbing and oxidation catalyst 11,and the air-fuel ratio of the exhaust gas flowing into the NO_(x)storing catalyst 12 becomes rich for a time longer than the time whenthe air-fuel ratio of the exhaust gas flowing into the HC adsorbing andoxidation catalyst 11 becomes rich, in the example shown in FIG. 8,several times the time.

In this way, in the present invention, by adsorbing and holding theparticulate fuel added from the fuel adding valve 14 in the HC adsorbingand oxidation catalyst 11 once, then making the adsorbed and heldparticulate fuel evaporate a little at a time from the HC adsorbing andoxidation catalyst 11, the air-fuel ratio of the exhaust gas flowinginto the NO_(x) storing catalyst 12 is made rich for a long time. Inthis case, to make the NO_(x) storing catalyst 12 release as large anamount of NO_(x) as possible, it is sufficient to make the time duringwhich the air-fuel ratio of the exhaust gas flowing into the NO_(x)storing catalyst 12 is rich longer. For this purpose, it becomesnecessary to increase the amount of fuel adsorbed and held at the HCadsorbing and oxidation catalyst 11 as much as possible.

Giving an example, it is learned that in a compression ignition internalcombustion engine where the amount of intake air per second becomes 10(g) at the time of engine low speed, low load operation, if injectingparticulate fuel from the fuel adding valve 14 for about 400 msec, theair-fuel ratio of the exhaust gas flowing into the NO_(x) storingcatalyst 12 will have a rich air-fuel ratio of about 14.0 over about 2seconds and that at that time, NO_(x) will be released well from theNO_(x) storing catalyst 12. At this time, the air-fuel ratio of theexhaust gas immediately downstream of the fuel adding valve 14, that is,the air-fuel ratio of the exhaust gas flowing into the HC adsorbing andoxidation catalyst 11, becomes a rich air-fuel ratio of about 4.4.

Explaining this in a bit more detail, in this compression ignitioninternal combustion engine, at the time of engine low speed, low loadoperation, the air-fuel ratio A/F is about 30. Therefore, since A/F=10(g/sec)/F=30, the amount of fuel injected becomes F 1/3 (g/sec). On theother hand, to produce a rich air-fuel ratio of 14, since A/F=10(g/sec)/F=14, 5/7 (g/sec) of fuel becomes necessary. Therefore, toproduce a rich air-fuel ratio of 14, the amount of additional fuel to beadded from the fuel adding valve 14 becomes 5/7 (g/sec)-1/3 (g/sec)=8/21(g/sec). To produce a rich air-fuel ratio of 14 over 2 seconds, it isnecessary to add 16/21 (g) of fuel from the fuel adding valve 14. Ifadding this fuel in 400 msec, the air-fuel ratio of the exhaust gas atthis time becomes about 4.4.

In this way, at the time of engine low speed, low load operation in thisinternal combustion engine, if trying to produce a rich air-fuel ratioof 14 over 2 seconds, it is necessary to supply 16/21 (g) of fuel fromthe fuel adding valve 14. In this case, if trying to supply this amountof fuel in a short time, for example, in 100 msec, it is necessary toraise the injection pressure of the fuel adding valve 14. However, ifraising the injection pressure of the fuel adding valve 14, the fuel ismade finer at the time of injection, so the amount of fuel which becomesa gas is increased and therefore the amount of fuel adsorbed at the HCadsorbing and oxidation catalyst 11 is reduced. That is, if the amountof fuel adsorbed on the HC adsorbing and oxidation catalyst 11decreases, the time during which the air-fuel ratio becomes rich becomessmaller. As opposed to this, when supplying 16/21 (g) of fuel, ifreducing the amount of supply per unit time, for example, if making thetime of addition of fuel from the fuel adding valve 14 1000 msec, theamount of evaporation of fuel from the HC adsorbing and oxidationcatalyst 11 per unit time becomes smaller and the air-fuel ratio of theexhaust gas is difficult to be made rich. FIG. 9 shows this.

That is, FIG. 9 shows the air-fuel ratio A/F of the exhaust gas flowinginto the HC adsorbing and oxidation catalyst 11, the temperature rise ΔTof the exhaust gas flowing out from the HC adsorbing and oxidationcatalyst 11, the exhausted HC amount G exhausted from the NO_(x) storingcatalyst 12, and the rich time of the exhaust gas flowing into theNO_(x) storing catalyst 12 when changing the fuel addition time τ (msec)from the fuel adding valve 14.

As explained above, if making the fuel addition time from the fueladding valve 14 shorter, the amount of fuel adsorbed at the HC adsorbingand oxidation catalyst 11 is reduced. As a result, the amount ofevaporation of fuel from the HC adsorbing and oxidation catalyst 11becomes smaller, so the oxidation action of the HC becomes weaker, thetemperature rise ΔT falls, and the rich time becomes shorter. Further,at this time, the amount of fuel carried off by the flow of exhaust gasin the fuel supplied from the fuel adding valve 14 increases, so theexhausted HC amount G increases.

On the other hand, if making the fuel addition time from the fuel addingvalve 14 longer, as explained above, the amount of fuel adsorbed perunit time at the HC adsorbing and oxidation catalyst 11 is reduced. As aresult, the amount of evaporation of fuel from the HC adsorbing andoxidation catalyst 11 becomes smaller, so the oxidation action of the HCbecomes weaker, the temperature rise ΔT falls, and the rich time becomesshorter. On the other hand, even after the action of release of NO_(x)from the NO_(x) storing catalyst 12 ends, HC continues to evaporate fromthe HC adsorbing and oxidation catalyst 11, so the exhausted HC amount Gincreases.

The fuel added when adding fuel from the fuel adding valve 14 isexhausted into the atmosphere, so that fuel is completely wasted.Therefore, it is necessary to suppress the amount of exhaust of theadded fuel into the atmosphere, that is, the exhausted HC amount G, toan allowable value G₀ or less. The exhausted HC amount G being theallowable value G₀ or less, if looked at differently, means that the HCis engaging in an oxidation reaction and oxygen is being sufficientlyconsumed. Therefore, the exhausted HC amount G being the allowable valueG₀ or less corresponds to the temperature rise ΔT being at least apredetermined setting ΔT₀.

That is, when adding fuel from the fuel adding valve 14, it is necessaryto determine the time τ of addition of the additional fuel so that theexhausted HC amount G becomes the allowable value G₀ or less andtemperature rise ΔT becomes the set value ΔT₀ or more. Therefore, inthis embodiment of the present invention, the time τ of addition of theadditional fuel is set to from about 100 (msec) to about 700 (msec). Ifexpressing this by the air-fuel ratio A/F, the air-fuel ratio A/F whenthe time τ of addition is 100 (msec) becomes about 1, while the air-fuelratio A/F when the time τ of addition is 700 (msec) becomes about 7, soin this embodiment of the present invention, at the time of engine lowspeed, low load operation, the amount of addition of particulate fueladded from the fuel adding valve 14 to make the NO_(x) storing catalyst12 release NO_(x) is set to an amount giving an air-fuel ratio of theexhaust gas flowing into the HC adsorbing and oxidation catalyst 11 ofabout 1 to about 7.

FIG. 10 shows the air-fuel ratio at the same locations as FIG. 8 at thetime of an engine high speed, high load operation. At the time of anengine high speed, high load operation, the temperature of the HCadsorbing and oxidation catalyst 11 becomes higher and the spatialvelocity of the exhaust gas flowing through the HC adsorbing andoxidation catalyst 11 becomes higher compared with the time of enginelow speed, low load operation, so, as will be understood from FIGS. 7(A)and (B), the amount of fuel which the HC adsorbing and oxidationcatalyst 11 can adsorb falls considerably. Therefore, as will beunderstood if comparing FIG. 10 and FIG. 8, the amount of fuel addedfrom the fuel adding valve 14 is made smaller at the time of engine highspeed, high load operation compared with the time of engine low speed,low load operation.

Note that as shown in FIG. 10, at the time of engine high speed, highload operation, the air-fuel ratio is about 20, so even if the fueladded is reduced, the air-fuel ratio of the exhaust gas can be maderich. However, the time during which the air-fuel ratio of the exhaustgas can be made rich becomes considerably shorter compared with the timeof engine low speed, low load operation.

FIG. 11(A) shows the amount of fuel AQ added from the fuel adding valve14 when NO_(x) should be released from the NO_(x) storing catalyst 12.The amount of fuel added becomes gradually smaller in the order of AQ₁,AQ₂, AQ₃, AQ₄, AQ₅, and AQ₆. Note that in FIG. 11(A), the ordinate TQshows the output torque, while the abscissa N shows the engine speed.Therefore, the amount of fuel AQ to be added becomes smaller the greaterthe output torque TQ, that is, the higher the temperature of the HCadsorbing and oxidation catalyst 11, while becomes smaller the higherthe engine speed N, that is, the greater the flow rate of the exhaustgas. The amount of fuel AQ to be added is stored in the form of a map asshown in FIG. 11(B) in advance in the ROM 32.

Next, the NO_(x) release control will be explained while referring toFIG. 12 and FIG. 13.

FIG. 12(A) shows the change in the NO_(x) amount ΣNOX stored in theNO_(x) storing catalyst 12 and the timing for making the air-fuel ratioA/F of the exhaust gas rich for release of NO_(x) at the time of enginelow speed, low load operation, while FIG. 12(B) shows the change in theNO_(x) amount ΣNOX stored in the NO_(x) storing catalyst 12 and thetiming for making the air-fuel ratio A/F of the exhaust gas rich forrelease of NO_(x) at the time of engine high speed, high load operation.

The amount of NO_(x) exhausted from the engine per unit time changes inaccordance with the engine operating state, therefore the amount ofNO_(x) stored in the NO_(x) storing catalyst 12 per unit time alsochanges in accordance with the engine operating state. In thisembodiment of the present invention, the amount of NO_(x) stored in theNO_(x) storing catalyst 12 per unit time is stored as a function of therequired torque TQ and the engine speed N in the form of a map shown inFIG. 13(A) in advance in the ROM 32. By cumulatively adding this NO_(x)amount NOXA, the NO_(x) amount ΣNOX stored in the NO_(x) storingcatalyst 12 is calculated.

On the other hand, in FIGS. 12(A) and (B), MAX indicates the maximumamount of NO_(x) which the NO_(x) storing catalyst 12 can store, whileNX indicates the allowable value of the amount of NO_(x) which can bemade to be stored in the NO_(x) storing catalyst 12. Therefore, as shownin FIGS. 12(A) and (B), when the NO_(x) amount ΣNOX reaches theallowable value NX, the air-fuel ratio A/F of the exhaust gas flowinginto the NO_(x) storing catalyst 12 is made temporarily rich and therebyNO_(x) is released from the NO_(x) storing catalyst 12.

As explained above, at the time of engine low speed, low load operation,the amount of fuel which the HC adsorbing and oxidation catalyst 11 canadsorb increases, so the amount of fuel added from the fuel adding valve14 is increased. If the amount of fuel added is increased in this way,the NO_(x) storing catalyst 12 can be made to release a large amount ofNO_(x). That is, in this case, even when the NO_(x) storing catalyst 12stores a large amount of NO_(x), all of the stored NO_(x) can bereleased, so, as shown in FIG. 12(A), the allowable value NX is made ahigh value, in the embodiment shown in FIG. 12(A), a value just slightlylower than the maximum NO_(x) stored amount.

As opposed to this, at the time of engine high speed, high loadoperation, the amount of fuel adsorbed by the HC adsorbing and oxidationcatalyst 11 decreases, so as explained above, the amount of fuel addedfrom the fuel adding valve 14 is reduced. If the amount of fuel added isreduced in this way, it is only possible to make the NO_(x) storingcatalyst 12 release a small amount of NO_(x). That is, in this case, itis necessary to release the stored NO_(x) after a small amount of NO_(x)is stored in the NO_(x) storing catalyst 12, so as shown in FIG. 12(B),the allowable value NX is made a considerably low value, in theembodiment shown in FIG. 12(B), a value of ⅓ or less of the allowablevalue NX at the time of engine low speed, low load operation shown inFIG. 12(A).

FIG. 13(B) shows the allowable value NX set in accordance with theengine operating state. In FIG. 13(B), the allowable value NX becomesgradually smaller in the order of NX₁, NX₂, NX₃, NX₄, NX₅, and NX₆. Notethat the allowable value NX shown in FIG. 13(B) is stored in the form ofa map as shown in FIG. 13(C) in advance in the ROM 32.

In this way, the higher the engine load or the higher the engine speed,the lower the allowable value NX, so to make the NO_(x) storing catalyst12 release NO_(x), the higher the engine load or the higher the enginespeed N, the higher the frequency of addition of particulate fuel fromthe fuel adding valve 14. That is, as shown in FIGS. 12(A) and (B), atthe time of engine high speed, high load operation, the frequency ofaddition of particulate fuel becomes considerably higher compared withthe time of engine low speed, low load operation.

On the other hand, the particulate matter contained in the exhaust gasis trapped on the particulate filter 12 a carrying the NO_(x) storingcatalyst 12 and successively oxidized. However, if the amount of theparticulate matter trapped becomes greater than the amount of theparticulate matter oxidized, the particulate matter will graduallydeposit on the particulate filter 12 a. In this case, if the depositionof particulate matter increases, a drop in the engine output will end upbeing invited. Therefore, when the deposition of particulate matterincreases, it is necessary to remove the deposited particulate matter.In this case, if raising the temperature of the particulate filter 12 aunder an excess of air to about 600° C., the deposited particulatematter is oxidized and removed.

Therefore, in this embodiment of the present invention, when the amountof the particulate matter deposited on the particulate filter 12 aexceeds the allowable amount, the temperature of the particulate filter12 a is raised under a lean air-fuel ratio of the exhaust gas andthereby the deposited particulate matter is removed by oxidation.Specifically speaking, in this embodiment of the present invention, whenthe differential pressure ΔP before and after the particulate filter 12a detected by the differential pressure sensor 23 exceeds the allowablevalue PX, it is judged that the amount of deposited particulate matterhas exceeded the allowable amount. At that time, the air-fuel ratio ofthe exhaust gas flowing into the particulate filter 12 a is maintainedlean, fuel is added from the fuel adding valve 14, and the heat ofoxidation reaction of the fuel added raises the temperature of theparticulate filter 12 a in temperature raising control.

FIG. 14 shows the exhaust purification processing routine.

Referring to FIG. 14, first, at step 100, the amount NOXA of NO_(x)stored per unit time is calculated from the map shown in FIG. 13(A).Next, at step 101, this NOXA is added to the NO_(x) amount ΣNOX storedin the NO_(x) storing catalyst 12. Next, at step 102, the allowablevalue NX is calculated from the map shown in FIG. 13(C). Next, at step103, it is judged if the stored NO_(x) amount ΣNOX has exceeded theallowable value NX. When ΣNOX>NX, the routine proceeds to step 104,where processing is performed to add fuel from the fuel adding valve 14.A basic example of this fuel addition processing is shown in FIG. 15.Two examples of correction of the amount of addition are shown in FIG.16 and FIG. 17. Next, at step 105, the differential pressure sensor 23is used to detect the differential pressure ΔP before and after theparticulate filter 12 a. Next, at step 106, it is judged if thedifferential pressure ΔP has exceeded the allowable value PX. WhenΔP>PX, the routine proceeds to step 107, where temperature raisingcontrol of the particulate filter 12 a is performed.

FIG. 15 shows the basic fuel addition processing when NO_(x) should bereleased from the NO_(x) storing catalyst 12. In this basic fueladdition processing, first, at step 150, the amount of fuel AQ to beadded is calculated from the map shown in FIG. 11(B), then at step 151,the fuel, that is, diesel oil, of the amount AQ calculated from the mapis added from the fuel adding valve 14.

However, if the air-fuel ratio of the exhaust gas flowing into theNO_(x) storing catalyst 12 does not become rich due to some sort ofreason even if adding an amount AQ of fuel predetermined in accordancewith the engine operating state, the NO_(x) storing catalyst 12 will notrelease NO_(x). Therefore, in this case, it is preferable to correct theamount of fuel added from the fuel adding valve 14 so that the air-fuelratio of the exhaust gas flowing into the NO_(x) storing catalyst 12becomes rich. Therefore, in another embodiment of the present invention,provision is made of judging means for judging if the air-fuel ratio ofthe exhaust gas flowing out from the HC adsorbing and oxidation catalyst11 has become rich when particulate fuel is added into the exhaust gasfor making the NO_(x) storing catalyst 12 release NO_(x). When NO_(x)should be released from the NO_(x) storing catalyst 12, the amount offuel required for making the air-fuel ratio of the exhaust gas flowingout from the HC adsorbing and oxidation catalyst 11 rich is addedaccording to judgment by this judging means.

As already explained based on FIG. 9, when the air-fuel ratio of theexhaust gas flowing into the NO_(x) storing catalyst 12 is rich, thetemperature rise ΔT of the exhaust gas passing through the HC adsorbingand oxidation catalyst 11 becomes the reference value ΔT₀ or more.Therefore, in the first example shown in FIG. 1, when the temperaturedifference between the temperature detected by the temperature sensor 21and the temperature detected by the temperature sensor 22, that is, thetemperature rise ΔT, has exceeded the reference value ΔT₀, it is judgedthat the air-fuel ratio of the exhaust gas flowing out from the HCadsorbing and oxidation catalyst 11 has become rich.

On the other hand, as shown in FIGS. 8(B) and (C) or FIGS. 10(B) and(C), when the air-fuel ratio A/F of the exhaust gas flowing out from theHC adsorbing and oxidation catalyst 11 becomes just slightly rich, theair-fuel ratio A/F of the exhaust gas flowing out from the NO_(x)storing catalyst 12 becomes substantially the stoichiometric air-fuelratio. Therefore, in the second example shown in FIG. 2, the air-fuelratio sensor 26 is provided so as to detect the air-fuel ratio of theexhaust gas flowing out from the NO_(x) storing catalyst 12. When theair-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor26 is substantially the stoichiometric air-fuel ratio, it is judged thatthe air-fuel ratio of the exhaust gas flowing out from the HC adsorbingand oxidation catalyst 11 is rich.

Note that in the embodiment shown in FIG. 1 and FIG. 2, when it isjudged that the air-fuel ratio of the exhaust gas flowing out from theHC adsorbing and oxidation catalyst 11 is not rich, the amount ofparticulate fuel added from the fuel adding valve 14 is increased. Theaction of increase of the amount of fuel added is performed for exampleby increasing the pulse like fuel addition time.

On the other hand, when it is judged that the air-fuel ratio of theexhaust gas flowing out from the HC adsorbing and oxidation catalyst 11is not rich, the action of addition of fuel from the fuel adding valve14 is already completed. Therefore, at this time, when it is next judgedthat the NO_(x) storing catalyst 12 should release NO_(x), the amount ofparticulate fuel added from the fuel adding valve 14 is increased.

FIG. 16 shows the fuel addition control in the case of using thetemperature sensors 21 and 22 to detect the temperature rise ΔT of theexhaust gas passing through the HC adsorbing and oxidation catalyst 11in FIG. 1.

Referring to FIG. 16, first, at step 200, the amount of fuel added AQ iscalculated from the map shown in FIG. 11(B). Next, at step 201, theamount of fuel added AQ is multiplied with a correction coefficient K tocalculate the final amount of fuel added AQ (=AQ·K). Next, at step 202,fuel, that is, diesel oil, is added from the fuel adding valve 14 inaccordance with the final amount of fuel added AQ.

Next, at step 203, the elapse of a certain time from the addition of thefuel is awaited. When that certain time has elapsed, the routineproceeds to step 204, where it is judged based on the output signals ofthe temperature signals 21 and 22 if the temperature rise ΔT is lowerthan a reference value ΔT₀. When it is judged that ΔT≧ΔT₀, the routineproceeds to step 207, where ΣNOX is cleared, then the processing cycleis ended. When it is judged that ΔT<ΔT₀, the routine proceeds to step205.

At step 205, the correction coefficient K is increased by a certainvalue ΔK, then at step 206 the elapse of a predetermined wait time, thatis, the consumption of the added fuel, is awaited. When the wait timeelapses, the routine proceeds through step 200 to step 201 and step 202,whereby a larger amount of fuel than the previous time is added.

FIG. 17 shows the fuel addition control in the case of detecting theair-fuel ratio A/F of the exhaust gas flowing out from the NO_(x)storing catalyst 12 by an air-fuel ratio sensor 26 as shown in FIG. 2.

In the routine shown in FIG. 17, the only difference from the routineshown in FIG. 16 is step 204′. Therefore, only step 204′ of the routineshown in FIG. 17 will be explained.

Referring to FIG. 17, at step 204′, it is judged based on the outputsignal of the air-fuel ratio sensor 26 whether the air-fuel ratio A/F ofthe exhaust gas flowing out from the NO_(x) storing catalyst 12 is aboutthe stoichiometric air-fuel ratio. When it is judged that it is aboutthe stoichiometric air-fuel ratio, the routine proceeds to step 207,while when it is judged that it is not about the stoichiometric air-fuelratio, the routine proceeds to step 205.

1. An exhaust purification device for a compression ignition typeinternal combustion engine comprising fuel adding means for addingparticulate fuel into exhaust gas, an HC adsorbing and oxidationcatalyst arranged in an engine exhaust passage downstream of the fueladding means for adsorbing and oxidizing hydrocarbons contained in theexhaust gas, and an NO_(x) storing catalyst arranged in the engineexhaust passage downstream of the HC adsorbing and oxidation catalystfor storing NO_(x) contained in the exhaust gas when the air-fuel ratioof the inflowing exhaust gas is lean and releasing the stored NO_(x)when the air-fuel ratio of the inflowing exhaust gas becomes thestoichiometric air-fuel ratio or rich, wherein particulate fuel is addedfrom the fuel adding means when making the air-fuel ratio of the exhaustgas flowing into the NO_(x) storing catalyst rich to make the NO_(x)storing catalyst release NO_(x), the amount of addition of particulatefuel at this time is set to an amount whereby the air-fuel ratio of theexhaust gas flowing into the HC absorbing and oxidation catalyst becomesa rich air-fuel ratio smaller than the rich air-fuel ratio when flowinginto the NO_(x) storing catalyst, and after the added particulate fuelis adsorbed at the HC adsorbing and oxidation catalyst, and the majorityof the adsorbed fuel is oxidized in the HC adsorbing and oxidationcatalyst and the air-fuel ratio of the exhaust gas flowing into theNO_(x) storing catalyst is made rich over a longer period than when theair-fuel ratio of the exhaust gas flowing into the HC adsorbing andoxidation catalyst is made rich.
 2. An exhaust purification device asset forth in claim 1, wherein an amount of particulate fuel to be addedfrom said fuel adding means for making the NO_(x) storing catalystrelease NO_(x) is set to an amount giving an air-fuel ratio of theexhaust gas flowing into the HC adsorbing and oxidation catalyst about 1to about 7 at the time of engine low speed, low load operation.
 3. Anexhaust purification device as set forth in claim 1, wherein the amountof particulate fuel added from said fuel adding means for making theNO_(x) storing catalyst release NO_(x) is reduced the higher thetemperature of the HC adsorbing and oxidation catalyst.
 4. An exhaustpurification device as set forth in claim 1, wherein the amount ofaddition of particulate fuel from said fuel adding means for making theNO_(x) storing catalyst release NO_(x) is reduced the greater the flowrate of the exhaust gas.
 5. An exhaust purification device as set forthin claim 1, wherein the amount of addition of particulate fuel from saidfuel adding means for making the NO_(x) storing catalyst release NO_(x)is made smaller at the time of engine high speed, high load operationcompared with the time of engine low speed, low load operation.
 6. Anexhaust purification device as set forth in claim 1, wherein thefrequency of addition of particulate fuel from said fuel adding meansfor making the NO_(x) storing catalyst release NO_(x) is higher thehigher the engine load.
 7. An exhaust purification device as set forthin claim 1, wherein particulate fuel is added from said fuel addingmeans to make the NO_(x) storing catalyst release NO_(x) when the amountof NO_(x) stored in the NO_(x) storing catalyst exceeds an allowablevalue, and the allowable value is made lower the higher the engine load.8. An exhaust purification device as set forth in claim 1, wherein aprecious metal catalyst is carried on a base of said HC adsorbing andoxidation catalyst.
 9. An exhaust purification device as set forth inclaim 1, wherein a base of said HC adsorbing and oxidation catalystincludes zeolite.
 10. An exhaust purification device as set forth inclaim 1, where said device comprises judging means for judging if theair-fuel ratio of the exhaust gas flowing out from the HC adsorbing andoxidation catalyst has become rich when particulate fuel is added intothe exhaust gas to make the NO_(x) storing catalyst release NO_(x), andsaid fuel adding means adds fuel of the amount necessary for making theair-fuel ratio of the exhaust gas flowing out from the HC adsorbing andoxidation catalyst rich in accordance with the judgment of said judgingmeans when making the NO_(x) storing catalyst release NO_(x).
 11. Anexhaust purification device as set forth in claim 10, whereintemperature sensors able to detect a temperature rise of exhaust gasflowing out from the HC adsorbing and oxidation catalyst are arranged inthe engine exhaust passage, and said judging means judges that theair-fuel ratio of the exhaust gas flowing out from the HC adsorbing andoxidation catalyst has become rich when said temperature rise exceeds areference value.
 12. An exhaust purification device as set forth inclaim 10, wherein an air-fuel ratio sensor able to detect the air-fuelratio of the exhaust gas flowing out from the NO_(x) storing catalyst isarranged in the engine exhaust passage downstream of the NO_(x) storingcatalyst, and said judging means judges that the air-fuel ratio of theexhaust gas flowing out from the HC adsorbing and oxidation catalyst hasbecome rich when the air-fuel ratio of the exhaust gas detected by theair-fuel ratio sensor is substantially the stoichiometric air-fuelratio.
 13. An exhaust purification device as set forth in claim 11,wherein when said judging means judges that the air-fuel ratio of theexhaust gas flowing out from the HC adsorbing and oxidation catalyst isnot rich, said fuel adding means increases the amount of particulatefuel added from the fuel adding means.
 14. An exhaust purificationdevice as set forth in claim 13, wherein when said judging means judgesthat the air-fuel ratio of the exhaust gas flowing out from the HCadsorbing and oxidation catalyst is not rich, said fuel adding meansincreases the amount of particulate fuel added from the fuel addingmeans when it is next judged that NO_(x) should be released from theNO_(x) storing catalyst.
 15. An exhaust purification device as set forthin claim 1, wherein the NO_(x) storing catalyst is carried on aparticulate filter for trapping and oxidizing particulate mattercontained in the exhaust gas.
 16. An exhaust purification device as setforth in claim 15, wherein raises the temperature of the particulatefilter is raised under a lean air-fuel ratio of the exhaust gas when theamount of particulate matter deposited on the particulate filter exceedsan allowable amount and thereby the deposited particulate matter isremoved by oxidation.
 17. An exhaust purification device as set forth inclaim 12, wherein when said judging means judges that the air-fuel ratioof the exhaust gas flowing out from the HC adsorbing and oxidationcatalyst is not rich, said fuel adding means increases the amount ofparticulate fuel added from the fuel adding means